It has to meet both of the above requirements, because I'll be growing the root veggies in containers.

Although the soil where I live is sandy (which I've heard is the type of soil root veggies grow best in) it's nutrient-poor.

Trying to emulate a soil from the ground (like what you'd
use for a plot in the ground) won't work for container
gardening. I tried to do that, and it was a disaster.

Please don't suggest find a soil mixture on the internet.
I've tried that and although I've found loads of info
about how to grow root veggies I have found nothing
about a soil suitable specifically for root veggies.

This is just preemptive. I'm not assuming that this is what ppl are going to say who post on the thread, it's
just clarification if you do suggest this advice.

I'm not trying to make accusations by what I just said,
so sorry if it comes off that way.

Use the 'search the forum' link at the top of the Container Gardening Forum for a more focused search of your topic of choice.